<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            position: absolute;
            left: 0;
            width: 100px;
            height: 100px;
            background-color: red;
        }
    </style>
</head>
<body>
    <div></div>
    <script>
        //一定要加定位
        var div=document.querySelector('div');
        var timer= setInterval(function(){
            if(div.offsetLeft>=400){
                //停止动画 停止定时器
                clearInterval(timer);
            }
            div.style.left=div.offsetLeft+1+'px';
        },30)


    </script>
</body>
</html>